1. Field of the Invention
This invention relates to transfer printing of sheet metal and in particular to a method of applying a pigmented base coat and optionally a clear topcoat over the pigmented base coat, transfer printing an image on the pigmented base coat or the topcoat, and then applying a protective overcoat over the transfer printed layer or coating.
2. Background Art
Transfer printing of inks or dyes into a variety of articles such as shirts, mugs, plastic articles and plastic coated substrates is well known in the art. Dispersible dye crystals or inks are printed in reverse images in transfer paper which is to be used to transfer print the article to be decorated. The article to be decorated is usually made of plastic or has a plastic or polymer coating on it into which the dyes are transferred. However, some transfer printing can be effective with most synthetic materials including the fibers in shirts.
Most transfer printing is referred to as sublimation printing in which the dyes are said to sublimate under heat and pressure to be driven into any receptive substrate that is put in contact with the transfer paper. Such sublimation printing was developed circa 1969 and has been used extensively to print many articles including plastic coated metal substrates.
Transfer printing also includes a melt printing process which is as described in several patents and patent applications including U.S. Pat. Nos. 4,587,155; 4,670,084; 4,668,239 and Published Application WO 92/21514. According to U.S. Pat. No. 4,587,155, the desired dye image is transferred from the paper to the substrate by heating the dye to a temperature above its melting point but below its vaporization temperature so that the dye will diffuse into the softened plastic substrate. Published Application WO 92/21514 describes melt printing of planar metal base members such as aluminum, steel or the like which have been coated on at least one planar surface with a melt printable layer of softenable, dye-permeable, thermoplastic or thermoset material such as polyethylene terephthalate, polybutylene terephthalate or other thermoplastic polyesters, polycarbonates, nylons and the like. Application WO 92/21514 further describes bilayer coatings of thermoplastic or thermoset materials including a base coat optimally provided with a pigment and a second layer that can comprise a clear resin. The application states that neither the composition nor the thickness of either the substrate or the printable plastic layers is critical.
Canadian Patent 1,108,929 describes a process for applying designs to unsealed, anodized aluminum which is continuously heated to a temperature at which colored components of ink will sublime. The heated strip is fed into contact with an ink carrying web so the ink is heated and transferred into the anodized aluminum surface. The decorated aluminum strip is then cooled with water and passed through a sealing bath filled with a sealing solution such as buffered aqueous nickel.
An improved process is needed for coating metal substrates for transfer printing to produce decorated sheets having consistent bright coloring with little or no texturing of the surface of the sheet. The process should produce decorated sheet metal that can be subsequently formed into shaped articles such as formed control panels or three dimensional pictures and signs without cracking or crazing of the coatings on the sheet metal. Coatings which have typically been used on transfer printed metal substrates cannot withstand the stringent forming requirements which include required bends with radii equal to approximately twice the metal thickness or more. Prior art coatings have also had poor resistance to ultraviolet light (UV) exposure and have frequently stained when exposed to washing aids such as bleach and solvent-based spot removers or to food products and alkalis. A process is needed for producing a thin, clear coating which has a uniform thickness with virtually no pin holes or other defects which might interfere with transfer printing including sublimation printing and melt printing. Improved transfer printed metal sheets are needed which are suitable for manufacture into a variety of products such as control panels, one-piece three-dimensional pictures and frames, and signs.